The present invention relates to electrochromic display devices and, more particularly, to a method for preparing display electrode layers in electrochromic display devices.
The electrochromic display devices are of the type using electrochromic material which undergoes reversible color changes upon the application of an electric field. In connection with an example of electrode structures in such display devices, see Nakauchi et al U.S. Pat. No. 4,253,741 issued Mar. 3, 1981 and entitled "Electrochromic Display Device", for example.
FIG. 1 shows a cross-sectional view of an ideal electrode structure at a display portion in the electrochromic display devices. The display electrode layers include a transparent substrate 1 made of glass or the like, a transparent, conductive film 2 made of In.sub.2 O.sub.3 or the like, an insulating film 3 made of SiO.sub.2, Si.sub.3 N.sub.4, MgF.sub.2 or the like, and an electrochromic film 4 made of WO.sub.3 or the like.
It is preferable that the transparent, conductive film 2 is covered by either of the insulating film 3 and the electrochromic film 4 in such a manner that the insulating film 3 neither overlaps with nor separates from the electrochromic film 4.
Conventionally, the display electrode layers are prepared with etching techniques using lift-off method. In this process, directly on the electrochromic film 4, an organic resist is printed in which heat is applied to obtain the insulating film 3. Hence, the electrochromic film 4 may react upon the organic resist coating.
The organic resist coating as called herein is featured in that the principal constituent thereof is a resin material, the organic resist is baked at about 80.degree. C., and it can be removed with an organic solvent. The resist may be carbonized when heated above about 150.degree. C. although the carbonization degree depends on the type of organic resist.
Upon being heated, the electrochomic film 4 may contract to thereby change its thickness or may be contaminated. Since the electrochromic film 4 may be annealed with heat applied to thereby crystallize, the display characteristics of the film 4 thus prepared are reduced. Unless heat is applied to prepare the insulating film 3, the insulating properties of the insulating film 3 are worse.
Therefore, it is desired that the display electrode layers be prepared without damaging their electro-optical properties.